The invention relates to a variable guide blade cascade for an exhaust-gas turbocharger of an internal combustion engine, the turbine stage of which comprises at least one spiral diagonal flow with a mouth region in the form of an annular nozzle, which opens onto a rotor at the turbine stage and in which a variable guide blade cascade having guide blades is arranged.
A variable guide blade cascade for an exhaust-gas turbocharger of an internal-combustion engine of the relevant generic type is already known from German Patent Specification 3,427,715.
The exhaust-gas turbocharger possesses a double-flow spiral guide channel, one flow opening radially and the other flow diagonally onto the rotor. In the mouth region of each of the two flows opening onto the rotor are located variable guide blade cascades with undivided guide blades arranged on its circumference, each guide blade being mounted rotatably about a longitudinal guide-blade axis.
Furthermore, German unexamined published patent application 2,843,202 discloses an exhaust-gas turbocharger with a radial wheel which is surrounded by a spiral guide channel. Arranged in an axially fixed manner in the mouth region of the guide channel is an annular partition wall having nozzle-shaped perforations which are distributed on its circumference and which are inclined out of the direction tangential to the radial wheel towards the axis of the radial wheel. The annular partition wall is preceded by a circumferentially displaceable diaphragm, by means of which the cross-section of the nozzle-shaped perforations can be adjusted.
Hitherto known variable guide blade cascades of exhaust-gas turbochargers of the relevant generic type have a relatively complicated construction, since an adjustment mechanism is required for moving each individual guide blade of the guide blade cascade. Guide blade cascades of this type are therefore expensive and are operationally reliable only at a comparatively high outlay.
Furthermore, in the even of a pronounced reduction in the flow cross-section during the adjustment of the guide blades, considerable vibration problems occur on the guide blades as a result of the accelerated flow.
An object on which the invention is based is to design a guide blade cascade of the relevant generic type with simpler construction than that of known versions, at the same time with an increase in its operational reliability.
This object is achieved, according to the invention, by providing an arrangement wherein the turbine stage of which comprises at least one spiral diagonal flow with a mouth region in the form of an annular nozzle, which opens onto a rotor at the turbine stage and in which a variable guide blade cascade having guide blades is arranged.
One advantage of the arrangement according to the invention of the variable guide blade cascade is that an increase in operational reliability is achieved as a result of an appreciable reduction in the number of movable components of the guide blade cascade.
In preferred embodiments of the invention, the guide blades are divided along a plane extending parallel to a radial plane of the rotor, an arrangement which results in an especially simple and cost-effective version of the divided guide blade cascade.
In certain preferred embodiments of the invention, division of the guide blades take place respectively by means of two intersecting sectional faces, one sectional face being a plane parallel to a radial plane of the rotor and the other sectional face being approximately parallel to a skeleton face of the respective guide blade. In certain preferred embodiments of the invention, the guide-blade parts wherein the guide-blade parts of the stationary guide-blade cascade part are respectively formed by profiled segments, and wherein the guide-blade parts of the rotatable guide-blade cascade part have a guide-blade profile with guide-blade cutouts congruent to the stationary guide-blade parts. The guide-blade cutouts in the rotatable guide-blade cascade part are located on the delivery side of the guide blades, and wherein the guide-blade surface on the suction side of the guide blades is made continuous without a gap. With these arrangements with the guide blade cascade fully open, the dividing plane of the divided guide blade cascade is located on the delivery side of the guide blades which is at less risk of breakaway, since, as a result of the curvature of the delivery side of the guide blades, a flow breaking away in the region of their dividing plane tends to join the guide blades again downstream. The suction side of the guide blades which is at risk of breakaway is made continuous without a gap, in order, here, to prevent a local flow breakaway caused by a gap.
In certain preferred embodiments the throughflow cross-section of the flow channels of the guide blade cascade can be regulated in dependance on operating parameters of the internal-combustion engine by means of the rotatable guide-blade cascade part. By regulating the throughflow cross-section via the rotatable guide-blade cascade part the latter can also be used as an engine brake. With a suitable choice of the cascade, it can be designed so that, with the exception of gap streams, it can be blocked completely.
In exhaust-gas turbochargers, the turbine stage of which has a radial and a diagonal flow, the design according to certain preferred embodiments of the invention of the guide blade cascade is such that in a turbine stage which has a spiral radial flow in addition to the spiral diagonal flow, a divided guide blade cascade having divided guide blades is arranged in each of the two mouth regions of the flows the guide blade cascades each having a stationary guide-blade cascade part and a movable guide-blade cascade part assigned thereto and rotatable concentrically relative to the axis of rotation of the rotor, the rotatable guide-blade cascade parts being coupled rigidly. This design ensures an increase in the flexibility of the power regulation of the exhaust-gas turbocharger, at the same time with relatively simple construction and high operational reliability, in comparison with known exhaust-gas turbochargers.
The cascade parameters of the two guide blade cascades are coordinated with preferred operating points of the exhaust-gas turbocharger. The divided guide blade cascades can advantageously be set at preferred operating points of the exhaust-gas turbocharger by a coordination of their cascade parameters, for example by a suitable choice of the cascade division, of the blade profile, of the blade angle and of the position of the dividing face between the stationary and the rotatable guide-blade cascade part. Furthermore, the angle between the longitudinal blade axis and the longitudinal guide-blade cascade axis (so-called rake angle) can also be varied, with the result that the breakaway behavior of the exhaust-gas flow can be influenced in an advantageous way. By means of the cascade parameters, the throughflow cross-section of the divided guide blade cascades can also be adjusted in dependence of the rotary angle of the rotatable guide-blade cascade part.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.